In recent years, organic light-emitting diodes have been attracting attention for application to a planar light source and the like. In particular, organic light-emitting diodes utilizing phosphorescence have been actively studied since they exhibit higher luminous efficiency.
There are two types of methods for producing such organic light-emitting diodes; a method using a vacuum evaporation process and a method using a coating process. The method using the coating process enables increase in device area and cost reduction; however, it has been still strongly expected to improve the luminous efficiency.
Conventionally, J. Appl. Phys., 102, 091101 (2007), for example, describes luminous efficiency of an organic light-emitting diode produced with use of a coating process. For example, it discloses an organic light-emitting diode having a structure in which, on a glass substrate, an anode of ITO, a hole injection layer of PEDOT:PSS, an emission layer comprising a host material polyvinylcarbazol (PVK), a phosphorescent blue-emitting material FIrpic, and an electron transport material OXD-7, and a cathode of CsF/Al are formed in this order. The organic light-emitting diode aims to obtain higher luminous efficiency by using PVK with higher triplet energy among host materials, for which a coating process can be applied, in order to confine the triplet excited state of the phosphorescent blue-emitting material FIrpic. However, the efficiency of the organic light-emitting diode is not still satisfactory.